1. Field of the Invention
The present invention is concerned with novel liquid crystal compounds and mixtures for electro-optical devices which are operated according to the two-frequency matrix addressing procedure, as well as their use in such devices.
2. Description of the Prior Art
Liquid crystal indicating devices have been widely used to display alphanumeric information. In the past, the liquid crystal indicating devices used a direct or static drive procedure to control the display of information.
The customary, static operation of liquid crystal indicating devices however has been replaced by a so-called multiplex control. In this multiplex case there mainly is used an amplitude-selective multiplex procedure whereby maximum multiplex ratios of 1:8 to 1:10 have been attained. To achieve higher multiplex rates in the control of liquid crystal displays, a two-frequency matrix addressing procedure has been proposed (e.g., German Offenlegungsschriften No. 2,856,134 (Great Britain Pat. No. 2,013,014) and 2,907,940 (Great Britain Pat. No. 2,020,075)).
This two-frequency procedure makes use of the fact that the dielectric anisotropy (.DELTA..epsilon.) of nematic liquid crystals having a positive dielectric anisotropy (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;0,.epsilon..sub..parallel. signifying the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto) upon application of a low-frequency voltage is negative in the case of a high frequency voltage. More particularly, the value of .epsilon..sub..parallel. for nematic liquid crystals decreases with increased voltage frequency. This effect has been ascribed to the hindering of the rotation of the long axis of the liquid crystal molecule around the short axis of the liquid crystal molecule (M. Schadt, Mol. Cryst. Liq. Cryst. 66 (1981) 319-336). In contrast to the relationship of .epsilon..sub..parallel. to voltage frequency, the value of .epsilon..sub..perp. in nematic liquid crystals remains constant except in the microwave frequency range. For .epsilon..sub..perp., dispersion effects only appear in the microwave range because of the barely hindered rotation of the molecule around its longitudinal axis. In the frequency range which is of interest here, .epsilon..sub..perp. is therefore constant, while .epsilon..sub..parallel. and consequently also .DELTA..epsilon. are frequency-dependent. The dielectric relaxation frequency, at which .epsilon..sub..parallel. =.epsilon..sub..perp., is denoted in the technical terminology as the "cross-over frequency" (f.sub.c). The most common nematic liquid crystals generally have cross-over frequencies of about 100 kHz and above at room temperature (23.degree. C.)
For the operation of an indicating device according to the two-frequency procedure there are used two alternating current sources, whereby the frequency of one of these sources must lie above the cross-over frequency and the frequency of the other source must lie below the cross-over frequency. Moreover, the voltage ratio of the signals for the on-condition and the off-condition must lie above a certain value. The greater this voltage ratio is, the more lines can be portrayed, i.e. the greater is the multiplex rate.
In addition, the two-frequency procedure offers the advantage that not only the switching-on process, but also the switching-off process can be influenced directly by the application of a corresponding voltage, whereby an acceleration of the switching-off process is achieved. For example, in the case of a liquid crystal indicating element with a twisted nematic structure (rotation cell) the homogeneously orientated liquid crystal can be aligned in the field direction by the application of a voltage of low frequency (f&lt;f.sub.c) and can again be converted into the twisted, homogeneous orientation by the application of a voltage of high frequency (f&gt;f.sub.c).
Furthermore, with liquid crystal cells which are based on guest-host effects (Applied Physics Letters 13 (1968) 91; D. L. White and G. N. Taylor, J. Appl. Phys. 45 (1974) 4718 inter alia) and which use liquid crystals having positive dielectric anisotropy, it generally is only possible to have colourless image elements on a coloured background. This imaging effect occurs because the colouring substances which are usable mainly exhibit positive dichroism. By homeotropic wall orientation and control according to the two-frequency procedure there can now be produced with such liquid crystals coloured image elements (homogeneously orientated by the application of a voltage of high frequency) on a colourless background.
The two-frequency procedure disadvantageously has high energy consumption since not only the amplitude of the applied alternating voltage but also the frequency are high. To reduce the energy consumption, it is therefore important to maintain a low operating voltage. The cross-over frequency (fc) should for the same reason lie relatively low (thereby the capacitive losses are small).